This invention relates to surgical apparatus for surgically removing thin sections of tissue and, in particular, to a keratome for use in ophthalmological procedures.
The eye is a compound optic comprising a cornea (the transparent, outer layer) and a lens. Approximately seventy percent of the focusing of light in the eye is performed by the cornea; viz. approximately forty-two diopters. (A diopter is a standard unit of measurement and is the reciprocal of the focal length of a lens in meters.) The remainder, about eighteen diopters, is performed by the lens of the eye. In the average total refraction of sixty diopters, a variation of less than five diopters is considered normal variation among a population. An error of more than five diopters is considered pathological.
Dr. Jose Barraquer of Bogota, Columbia, suggested in 1949 that many problems with vision could be corrected by surgically changing the shape of the cornea; viz. its curvature. Dr. Barraquer first published in 1964 the results of a procedure to correct myopia (nearsightedness). The procedure took a thin section from the front of the cornea, centered above the pupil. He froze the section, shaped it with a cryogenic lathe, thawed it, and re-attached it to the eye. The equipment for performing this procedure is large, expensive and not entirely satisfactory. In general, progress in this area has been limited by lack of suitable equipment; in particular, by a lack of equipment for removing smooth, round tissue of the desired thickness from a cornea and, at the same time, leaving a smooth bed for the tissue to adhere to when it is replaced.
The tool used for making corneal slices is known as a keratome. Some keratomes resemble a tiny block plane having an overall length of about three centimeters. Unlike a plane, the blade in the keratome oscillates from side to side at high speed to slice the tissue.
FIG. 1 illustrates a keratome of the prior art. The keratome comprises an elongated body 11 having a slanted surface at one end thereof for receiving blade 12. Cam 14 engages a hole in blade 12 to drive blade 12 from side to side (into and out of the plane of the drawing). Apparatus (not shown) extending through channel 16 in head 18 drives coupling member 14. Suitable drive means can be electrical or pneumatic. Head 18 is held in place by collar 21, which is threaded on to head 18 and a portion of body 11. Head 18 and body 11 together form a slot for containing blade 12. The front end of the keratome has a vertically adjustable plate 23. The difference in height between the edge of adjustable plate 23 and the edge of blade 12 determines the thickness of the section removed from the cornea. Adjustment means 24 provides a very small movement of plate 23, which is biased by spring 25 to avoid backlash.
(As used herein, terms implying direction are for convenience in referring to the drawings, e.g. "front," "back," "vertical," otherwise the terms are of no significance.)
In use, the plate is adjusted to an appropriate height. A tool known as an suction ring is applied to the front of the eye, centered over the iris. Suction rings are made in different thicknesses to vary the diameter of the section removed from the cornea. When placed over the eye, the suction ring locates, confines, and slightly bulges the cornea, by raising the interocular pressure, to flatten the cornea against the plate for slicing by the keratome. The suction ring has a dovetail fitting for receiving the lower edges of the keratome, which has a corresponding dovetail shape.
The power is applied to the keratome, which causes the blade to oscillate at ten to twenty thousand cycles per minute. The keratome is slid through the dovetail in the suction ring, causing a section of tissue of the cornea to be removed and pass into interior chamber 27 of the keratome. The surgeon then recovers the tissue from the keratome and later replaces it on the cornea.
FIG. 2 is a simplified illustration of a cross-section of a portion of a human eye. Eye 30 comprises lens 31 separated from cornea 32 by anterior space 33. In the central region above lens 31, cornea 32 has a thickness of about five hundred twenty microns which increases toward the conjunctiva (toward the sides).
The cornea itself actually comprises five layers, the outer three of which are illustrated in FIG. 3. The outermost layer is known as the epithelium layer, denoted as layer 35 in FIG. 3, and is fifty to ninety microns thick. The Bowman layer, denoted as layer 36, separates the epithelium from the substantia propria or stroma, layer 37. The Bowman layer is about twelve microns thick. Layer 37 comprises most of the thickness of the cornea, four hundred to four hundred and fifty microns.
During keratomileusis in-situ for myopia, two sections are removed from the cornea. A first section is cut as illustrated in FIG. 2, yielding disk 38. The disk is not flat because the cornea is flattened somewhat prior to the cut being made. After the cut, the disk returns to a three dimensional shape. Bed 39 is the outer surface of the remaining cornea. A second cut is made to remove a second disk. Disc 38 is then re-attached. (Disk 38 contains the critical Bowman layer). The effect is to flatten or reduce the curvature of the cornea, correcting nearsightedness.
The requirement for smooth cuts with no rough edges nor bed is compounded when two cuts are made. A smooth bed is especially difficult to leave after the second cut because of the structure of the cornea. Specifically, the epithelium or outer layer of the cornea comprises lamellar (thin, plate-like) cells 41 (FIG. 3) on its outer surface and columnar cells 42 on its inner surface. The columnar cells are more pliable and tend to yield rather than be cut by a blade. Also, with the somewhat tougher outer layer removed, the softer, fibrous tissue in layer 37 is more difficult to cut smoothly. Prior to the present invention, no keratome has been available which is capable of making perfectly round, smooth, second cuts for this procedure.
The risk of having rough edges and an irregular periphery restricts the range of thicknesses of the cuts that can be made and, hence, the range of corrections that can be obtained because the cuts must be relatively thick. This makes the procedure useful only for severely myopic patients; e.g. where the desired correction is eight diopters or more. It is highly desirable to be able to cut thin, precise disks, which will enable the surgeon to use the procedure for correction of low and mid-myopia (two to three diopters and four to eight diopters). Further, keratomes of the prior art leave undesirable microscopic ridges and valleys across the surface of the cornea, even in the hands of highly skilled surgeons.
Whether one disk or two disks are cut, a deficiency with keratomes of the prior art is the inconsistency of the cuts; i.e. a given combination of keratome setting and suction ring did not yield consistent, predictable results. Instead, the shapes and thicknesses of the disks varied and the remaining beds on corneas were rough. Since the correction is relative to the thickness of the cut and the smoothness of the bed, a different instrument is necessary.
In view of the foregoing, it is therefore an object of the invention to provide a surgical tool for obtaining accurate, smooth cuts of delicate tissue.
Another object of the invention is to provide an improved keratome for enabling one to make cuts having smooth edges and surfaces.
A further object of the invention is to provide an improved keratome capable of consistently cutting smooth, round disks of the diameter and thickness desired by the surgeon to correct a greater range of myopia.
Another object of the invention is to provide a keratome capable of making smooth, round, cuts on a human cornea.
A further object of the present invention is to provide a keratome that can produce consistent results from device to device.